Terrorize them??? The Catholic Church isn’t a cult which hunts down those who leave and drags them back.
Her parents are acting with the agency of Christ the Good Shepherd. Remember him, the one who leaves 99 sheep alone so he can go out and seek one lost sheep and bring it back to the fold? (When Jesus told this parable, all the Jews understood exactly what he meant. A shepherd who left 99 sheep was crazy, and bound to be fired for his negligence. Jesus was setting a new example as shepherd of men.)
There is a difference here between the institutional Catholic Church and her members. The Church doesn’t assign clergy to go out and drag heretics back against their will. Indeed, the Church takes her evangelistic mission very seriously. The Church commissions whole orders of religious to go out into the world and preach the Gospel to the lost, the poor, the pagans, the Protestants, the atheists. The Gospel message must get through to some people. It is the seed scattered by the sower. The Church has a Great Commission to sow that seed among people of all nations.
The members of the Church also have an evangelistic mission. We cannot preach with the authority of the Magisterium, but we can witness our personal faith to others. Some of us are called to speak to Catholics - that is what we do on the Catholic Answers forums. Some of us are called to speak to atheists instead. Parents are first and foremost called to witness to their children, to nurture them in the True Faith, and to call them back if they stray. Take the example of faithful St. Monica. Wife of a pagan man, she did everything possible for her pagan son, who strayed from the faith and spent his young life on immorality. She prayed for his conversion. She didn’t give up. The OP’s parents shouldn’t give up. There comes a point where they should back off, and that is what they have vowed to do. They will not support their daughter in an invalid marriage. I am sure that they will continue to pray for her conversion and salvation. I am grateful that my parents also prayed in this way, and eventually their prayers were answered.